In order to realize precise air-fuel ratio control, it is necessary to decide an amount of injected fuel in regard to the amount of intake air actually supplied into the cylinder. To detect the amount of intake air, an air flow meter is usually arranged in the intake system. However, the air flow meter has a delay in its response and, thus, is not capable of correctly detecting the amount of the intake air during a transient condition of the engine. Accordingly, it has been proposed to calculate and estimate an amount of intake air even during a transient condition of the engine. (For example, refer to Japanese Patent Publication Nos. 2002-130039 and 2002-201998.)
In the estimating of the amount of intake air, by modeling the throttle valve, it is necessary to calculate the amount of intake air passing through the throttle valve on the basis of a difference between an intake air pressures upstream of the throttle valve and an intake air pressure downstream thereof. In the above-mentioned prior art, when an amount of intake air passing through the throttle valve is calculated, the intake air pressure downstream of the throttle valve, namely the intake pipe pressure is varied. However, the intake air pressure upstream of the throttle valve is fixed to the atmospheric pressure. Thus, a precise amount of intake air passing through the throttle valve cannot be calculated. Therefore, a precise amount of intake air supplied into the cylinder cannot be estimated.